When you open Excel worksheets from unknown sources, the Protected View feature in Excel protects you from potential risks. That's great, but to work on the document, you need to switch to edit mode.
How to switch to edit mode
Protected view is enabled by default. So, if you open a file that comes from a web site or that you received by email, you will be redirected to the Protected View.
The message bar appears at the top of the document, allowing you to enable editing. Click the "Activate editing" button.
The document is now read-only and some content is disabled.
Save the document to another location for effective editing.
Reasons to Use the Protected View
The Protected View protects you from malware, viruses, and threats that you may be exposed to when opening Excel files from unknown sources. These sources include a Web site, email attachments, and other insecure storage locations such as shared drives.
If you only view the table, you do not need edit mode and can easily work with the document in Protected View.
Can I disable the protected view?
Yes, but you should check with your IT team or network administrator beforehand as it exposes you to external threats.
To turn off Protected View, click File on the taskbar, and then choose Options. Then select the Trust Center category and click the Trust Center Settings button.
Select the "Protected View" category. 
Three settings can be disabled here: files from the Internet, insecure locations, or Outlook attachments. You can disable all three options if you do not want to use Protected View in the future.
Protected View is a useful feature to protect users from potential threats when opening documents from unknown sources. This is very often the case when working with Excel on mobile devices and accessing documents in public places.
In most cases, however, these tables are safe. Fortunately, Microsoft makes it relatively easy to switch to edit mode.